Add support for working file attachment comments.

Commenting on a file attachment now works much like diffs and screenshots,
with a comment dialog and a list of existing comments.

File attachments only have comments made on them as a whole.

We may want to change the UI a bit later, perhaps indicate draft comments
visually like we do with other comment flags. For now, though, this gives
us something we can test with.

Commented on a few different files. I was able to add and change the comment,
see existing comments made, and see them in the review.